Method of fabricating long-wavelength VCSEL and apparatus

ABSTRACT

A long-wavelength VCSEL, and method of fabrication, includes a long-wavelength active region epitaxially grown on a compatible substrate with a high heat conductivity DBR mirror stack metamorphically grown on the active region. A supporting substrate is bonded to the DBR mirror stack and the compatible substrate is removed. A second mirror stack, either a DBR or a dielectric mirror stack, is formed on the opposite surface of the active region. Preferably, an InP based active region is grown on an InP based substrate and an AlAs/GaAs based metamorphic DBR mirror stack is metamorphically grown on the active region. The supporting substrate may be either an InP based substrate bonded to the active region or a layer of plated metal, such as copper, silver, gold, nickel, aluminum, etc.

This application is a division, of application Ser. No. 09/642,359,filed 21 Aug. 2000, U.S. Pat. No. 6,628,685.

FIELD OF THE INVENTION

This invention relates to a method of fabricating a vertical cavitysurface emitting laser which is capable of emitting long-wavelengthlight and to the vertical cavity surface emitting laser.

BACKGROUND OF THE INVENTION

Vertical cavity surface emitting lasers (VCSELs) include first andsecond distributed Bragg reflectors (DBRS) formed on opposite sides ofan active area. The VCSEL can be driven or pumped electrically byforcing current through the active area or optically by supplying lightof a desired frequency to the active area. Typically, DBRs or mirrorstacks are formed of a material system generally consisting of twomaterials having different indices of refraction and being easilylattice matched to the other portions of the VCSEL. In conventionalVCSELs, convention material systems perform adequately.

However, new products are being developed requiring VCSELs which emitlight having long-wavelengths. VCSELs emitting light havinglong-wavelengths are of great interest in the optical telecommunicationsindustry. This long-wavelength light can be generated by using a VCSELhaving an InP based active region. When an InP based active region isused, however, the DBRs or mirror stacks lattice matched to thesupporting substrate and the active region do not provide enoughreflectivity for the VCSELs to operate because of the insignificantdifference in the refractive indices between the two DBR constituents.

Dielectric mirror stacks can be used for VCSEL applications, but theysuffer from poor thermal conductivity. Since the performance of theselong-wavelength materials is very sensitive to temperature, the thermalconductivity of the DBRs is very important.

Accordingly it is highly desirable to provide a method of fabricatinglong-wavelength VCSELs with good thermal conductivity.

It is an object of the present invention to provide new and improvedmethods of fabricating long-wavelength vertical cavity surface emittinglasers.

It is another object of the present invention to provide new andimproved methods of fabricating long-wavelength vertical cavity surfaceemitting lasers in which materials with good thermal conductivity andrefractive indices are used.

It is still another object of the present invention to provide new andimproved long-wavelength vertical cavity surface emitting lasers.

It is a further object of the present invention to provide new andimproved long-wavelength vertical cavity surface emitting lasersincorporating materials with good thermal conductivity and refractiveindices.

It is yet a further object of the present invention to provide new andimproved long-wavelength vertical cavity surface emitting lasers whichcan be either optically or electrically pumped.

SUMMARY OF THE INVENTION

A long-wavelength VCSEL is disclosed with a novel method of fabrication.The VCSEL includes a long-wavelength active region epitaxially grown ona compatible substrate with a high heat conductivity distributed Braggreflector (DBR) mirror stack metamorphically grown on the active region.A supporting substrate is bonded to the DBR mirror stack and thecompatible substrate is removed. A second mirror stack, either a DBR ora dielectric mirror stack, is formed on the opposite surface of theactive region. The supporting substrate can be, for example, a thickmetal layer deposited on the DBR or a second semiconductor type ofsubstrate. The DBR and second mirror stack are preferably formed ofmaterials with good thermal conductivity and refractive indices.

In a preferred embodiment, an indium phosphide (InP) active region isgrown on an InP based substrate and an AlAs/GaAs based metamorphic DBRmirror stack is epitaxially grown on the active region. AlAs/GaAs hasgood thermal conductivity and sufficiently different refractive indicesto produce a good mirror stack. The supporting substrate may be either amechanical InP based substrate bonded to the active region or a layer ofplated metal, such as copper, silver, gold, nickel, aluminum, etc. Theplated metal supporting substrate provides additional thermalconductivity for the VCSEL.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIGS. 1 through 5 are simplified sectional views illustrating sequentialsteps in a method of fabricating VCSELs in accordance with the presentinvention; and

FIGS. 6 through 8 are simplified sectional views illustrating sequentialsteps in another method of fabricating VCSELs in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIGS. 1 through 5, various steps are illustrated,sequentially, in a method of fabricating vertical cavity surfaceemitting lasers (VCSELs) in accordance with the present invention.Referring specifically to FIG. 1, a substrate 10 is provided which maybe, for example a semiconductor wafer or the like. A long-wavelengthactive region 11 is formed on the upper surface of substrate 10 in anywell known process. Generally, active region 11 includes one or morequantum well layers with barrier layers therebetween and cladding and/orspacer layers defining the upper and lower surfaces. As is understood bythose skilled in the art, active region 11 is formed with a thickness ofapproximately one wavelength to multiple wavelengths of the emittedlight.

In a preferred embodiment, active region 11 is based on an indiumphosphide (InP) material system to provide a long-wavelength activeregion. Further, substrate 10 preferably includes InP so that activeregion 11 can be conveniently epitaxially grown on the surface with thedesired crystal lattice matching. For reasons that will be explained inmore detail presently, a thin etch-stop layer (not shown) can also beincluded as a lower portion of active region 11. Generally, theetch-stop layer can be any convenient and compatible material with alarge differential etching capability relative to substrate 10.

Referring additionally to FIG. 2, a distributed Bragg reflector (DBR)mirror stack 12 is formed on the upper surface of active region 11. Asexplained briefly above, it is common in the prior art to epitaxiallygrow alternate layers of, for example, InGaAsP and InAlGaAs on an InPbased active region. The major problem with this type of DBR is that therefractive index difference is too small to provide good reflectivity.Dielectric mirror stacks can be used, but they suffer from poor thermalconductivity. It has been found that materials with good thermalconductivity and refractive indices can be metamorphically grown onlong-wavelength active region 11. In this context, the term “goodthermal conductivity” generally means a thermal conductivity at least asgood as the thermal conductivity of an AlAs/GaAs DBR.

In a specific example, substrate 10 is an InP based semiconductor waferand long-wavelength active region 11 is grown on substrate 10.Long-wavelength active region 11 includes, for example, one or morequantum well layers of InGaAsP with InP barrier layers therebetween.Cladding or spacer layers on opposed sides of the quantum well layersinclude, for example, InP. In this specific example, alternate layers ofAlAs and GaAs are grown metamorphically on active region 11 to form DBR12. As is understood by those skilled in the art, DBR 12 includes asufficient number of mirror pairs (e.g., 20 to 40) so as to provide ahigh reflectivity for light generated by active region 11.

Here it should be understood that “metamorphic growth” is a type ofepitaxial growth (e.g. by PCVD, MOCVD, PECVD, CVD, sputtering, etc.,) inwhich the crystal lattice of the grown material does not strictly matchthe lattice of the substrate. By metamorphically growing the grownmaterial, the lattice of the grown material gradually changes fromsimilar to the lattice of the substrate to the relaxed lattice of thegrown material. In this fashion, DBR materials with good thermalconductivity and large difference in index of refraction can beconveniently grown on a long-wavelength active region. Some examples ofpairs of material with good thermal conductivity and index of refractionwhich can be metamorphically grown on a long-wavelength active regionare: AlAs and GaAs; micro-crystalline silicon and microcrystallinesilicon carbide; and micro-crystalline silicon and micro-crystallinealuminum oxide. Here it should be noted that AlAs/GaAs is a specificexample of a metamorphically distributed Bragg reflector includinglayers of Al_(x)Ga_(1-x)As/Al_(y)Ga_(1-y)As, where x is in a range offrom approximately 0.5 to 1 and y is in a range of from approximately 0to 0.5.

Referring to FIG. 3, once DBR mirror stack 12 is completed a heatspreader is formed on the upper surface. Generally, the heat spreader issome metal with high heat conductivity, such as copper, silver, gold,nickel, aluminum, etc. In a preferred embodiment, the heat spreaderincludes a first thin layer 15 which may be, for example, vacuumdeposited or the like. Also, in this specific embodiment the VCSEL isdesigned for optical pumping and therefore an opening 16 is formed inlayer 15 as an inlet for light to be used in the optical pumping, orexciting of active region 11. Opening 16 can be formed in layer 15 bywell known masking techniques, selective deposition, etc.

With layer 15 formed on the surface of DBR mirror stack 12, additionalmetal 17 is plated onto layer 15, as illustrated in FIG. 4, using a wellknown metal plating process (e.g., electroplating, vacuum deposition, orthe like). Layer 15 is provided as a plating contact for electroplatingand/or to allow for selective plating of additional metal 17. Because Inthis preferred embodiment and for purposes of example only, layer 17 isselectively plated onto layer 15, a larger opening 18 is automaticallyformed by the plating process in layer 17. The selective plating mayhave to be done in multiple steps to achieve the required totalthickness (e.g.>100 □m) for both mechanical support and a small opticalaperture (e.g.<10 □m).

Referring additionally to FIG. 5, once layers 15 and 17 are completed toprovide a supporting substrate, substrate 10 is removed. As will beunderstood by those skilled in the art, substrate 10 can be removed bystandard etching techniques, grinding and etching, etc. To facilitatethe etching process, an etch-stop layer can be provided betweensubstrate 10 and active region 11, if desired. Such etch-stop layers arewell known in the art and will not be discussed further.

Upon the removal of substrate 10, exposing the other side of activeregion 11, a second mirror stack 20 is formed on the exposed surface ofactive region 11. Because most of the heat produced by the VCSEL isconducted away by the good thermal conductivity of DBR mirror stack 12and the heat spreader (i.e. layers 15 and 17), either a dielectricmirror stack can be deposited on the exposed surface of active region 11or the composite structure can be used to grow another metamorphic DBRmirror stack on the exposed surface of active region 11.

Generally, if the VCSEL is to be an optically pumped laser, mirror stack20 is most conveniently formed as a dielectric mirror stack. When theVCSEL is to be an electrically pumped laser, electrical contact isgenerally made to both sides of active region 11. Electrical contactthrough DBR mirror stack 12 can be provided by simply doping DBR mirrorstack 12 during growth. Electrical contact to the other side of activeregion 11 generally requires some form of electrical conductor betweenthe dielectric mirror stack and active region 11 (since a dielectricmirror stack is not electrically conductive) or doped metamorphic DBRmirror stacks on both sides of active region 11.

Generally, to define a lasing cavity for efficient operation of theVCSEL, some form of index guiding structure is used. In this specificembodiment, for example, index guiding structures can be formed bypatterning active region 11 after substrate 10 is removed and/or bypatterning mirror stack 20. As illustrated in FIG. 5 by cylindrical lineor wall 21 a lasing volume or cavity is defined within active area 11.Cylindrical line or wall 21 can be formed using a number of well knownmethods, including etching one or all of the portions (i.e. layers 11,12, and 20) outside of line 21, damaging the portion or portions so thatthey will not conduct light, or otherwise limiting the operation of theVCSEL to the volume within line 21. The index guiding structure used isalso generally used to separate a plurality of VCSELs fabricated on acommon substrate or wafer into individual wafers or arrays.

Turning now to FIGS. 6 through 8, several sequential steps areillustrated in another fabrication process of a long-wavelength VCSEL inaccordance with the present invention. In this method, the substrate,active region, and DBR mirror stack of FIG. 2 is used as the basis. InFIG. 6, components similar to those illustrated in FIG. 2 are designatedwith a similar number and all numbers have a prime added to indicate thedifferent embodiment. In this embodiment a substrate 25′ is bonded tothe upper exposed surface of DBR mirror stack 12′, rather thandepositing a heat spreader as in FIGS. 3 and 4. Further, substrate 10′is designated substrate #1 and substrate 25′ is designated substrate #2only for purposes of differentiating the two substrates.

In this preferred embodiment, DBR mirror stack 12′ is metamorphicallygrown on active region 11′ so that if, for example, substrate 10′ is InPbased and active region 11′ is InP based, then substrate 25′ could beInP based and would alleviate any thermal mismatch problems becausesubstrate 25′ is essentially bonded to an InP based structure. That is,after the metamorphic growth, substrate 25′ is thermally bonded to amechanical InP based substrate. This process can be done with large sizewafers because there is no thermal mismatch between substrate 10′ andsubstrate 25′. Once substrate 25′ is bonded to the structure, substrate10′ is removed (see FIG. 7) to expose the other surface of active region11′.

Upon the removal of substrate 10′, exposing the other side of activeregion 11′, a second mirror stack 26′ is formed on the exposed surfaceof active region 11′. Because most of the heat produced by the VCSEL isconducted away by the good thermal conductivity of DBR mirror stack 12′,either a dielectric mirror stack can be deposited on the exposed surfaceof active region 11′ or the composite structure can be used to growanother metamorphic DBR mirror stack on the exposed surface of activeregion 11′.

Generally, as described above, to define a lasing cavity for efficientoperation of the VCSEL, some form of index guiding structure is used. Inthis specific embodiment, for example, index guiding structures can beformed by patterning active region 11′ after substrate 10′ is removedand before mirror stack 26′ is deposited. Index guiding structures canalso be formed by patterning mirror stack 25′ during deposition orgrowth. As illustrated in FIG. 8 by cylindrical line or wall 28′ alasing volume or cavity is defined within active area 11′. Cylindricalline or wall 28′ can be formed using a number of well known methods,including etching one or all of the portions (i.e. layers 11′, 12′, and26′) outside of line 28′, damaging the portion or portions so that theywill not conduct light, or otherwise limiting the operation of the VCSELto the volume within line 28′.

Thus, new and improved methods of fabricating long-wavelength verticalcavity surface emitting lasers have been disclosed in which materialswith good thermal conductivity and refractive indices are used. Also,substrates bonded to the VCSEL structure during fabrication arethermally matched to the structure so that thermal mismatch problems areavoided and large size wafers can be used. Further, new and improvedlong-wavelength vertical cavity surface emitting lasers are disclosedincorporating materials with good thermal conductivity and refractiveindices. The good thermal conductivity material is used in a structurethat provides good heat sinking capabilities. The new and improvedlong-wavelength vertical cavity surface emitting lasers can be eitheroptically or electrically pumped and either can be fabricated using wellknown semiconductor processes.

While we have shown and described specific embodiments of the presentinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular forms shown and we intend inthe appended claims to cover all modifications that do not depart fromthe spirit and scope of this invention.

1. A method of fabricating a long-wavelength vertical cavity surfaceemitting laser comprising the steps of: depositing a long wave-lengthactive region on a compatible substrate, the long wave-length activeregion having a first major surface; depositing a first mirror stack onthe first major surface of the long wave-length active region so as todefine a major surface of the first mirror stack; affixing a supportingsubstrate to the major surface of the first mirror stack; removing thecompatible substrate to expose an opposed second major surface of thelong wave-length active region; and depositing a second mirror stack onthe second major surface of the long wave-length active region; whereinthe step of depositing the long wave-length active region on thecompatible substrate includes epitaxially growing an indium phosphidebased active region on an indium phosphide based substrate.
 2. A methodof fabricating a long-wavelength vertical cavity surface emitting laseras claimed in claim 1 wherein the step of depositing the first mirrorstack includes depositing layers of material with good thermalconductivity.
 3. A method of fabricating a long-wavelength verticalcavity surface emitting laser as claimed in claim 1 wherein the step ofdepositing layers of material with good thermal conductivity includesdepositing layers of material with a thermal conductivity comparable toa lattice matched semiconductor distributed Bragg reflector.
 4. A methodof fabricating a long-wavelength vertical cavity surface emitting laseras claimed in claim 1 wherein the step of affixing the supportingsubstrate to the major surface of the first mirror stack includesbonding a supporting substrate approximately thermal-expansion matchedto the first mirror stack.
 5. A method of fabricating a long-wavelengthvertical cavity surface emitting laser as claimed in claim 4 wherein thestep of bonding a supporting substrate includes bonding a mechanical InPsubstrate to the first mirror stack.
 6. A method of fabricating along-wavelength vertical cavity surface emitting laser as claimed inclaim 1 wherein the step of depositing the second mirror stack includesdepositing one of a distributed Bragg reflector and a dielectric mirrorstack.
 7. A method of fabricating a long-wavelength vertical cavitysurface emitting laser as claimed in claim 6 wherein the step ofdepositing the second mirror stack includes depositing layers ofmaterial with good temperature conductivity.
 8. A method of fabricatinga long-wavelength vertical cavity surface emitting laser comprising thesteps of: depositing a long wave-length active region on a compatiblesubstrate, the long wave-length active region having a first majorsurface; depositing a first mirror stack on the first major surface ofthe long wave-length active region so as to define a major surface ofthe first mirror stack; affixing a supporting substrate to the majorsurface of the first mirror stack; removing the compatible substrate toexpose an opposed second major surface of the long wave-length activeregion; and depositing a second mirror stack on the second major surfaceof the long wave-length active region; wherein the step of depositing afirst mirror stack includes metamorphically growing a distributed Braggreflector on the first major surface of the long wave-length activeregion; and wherein the step of metamorphically growing a distributedBragg reflector includes metamorphically growing alternate layers ofAl_(x)Ga_(1-x)As and Al_(y)Ga_(1-y)As, where x in a range of fromapproximately 0.5 to 1 and y is in a range of from approximately 0 to0.5.
 9. A method of fabricating a long-wavelength vertical cavitysurface emitting laser as claimed in claim 8 wherein the step ofmetamorphically growing alternate layers of Al_(x)Ga_(1-x)As andAl_(y)Ga_(1-y)As includes growing an AlAs/GaAs distributed Braggreflector on an InP long wave-length active region.
 10. A method offabricating a long-wavelength vertical cavity surface emitting lasercomprising the steps of: depositing a long wave-length active region ona compatible substrate, the long wave-length active region having afirst major surface; depositing a first mirror stack on the first majorsurface of the long wave-length active region so as to define a majorsurface of the first mirror stack; affixing a supporting substrate tothe major surface of the first mirror stack; removing the compatiblesubstrate to expose an opposed second major surface of the longwave-length active region; and depositing a second mirror stack on thesecond major surface of the long wave-length active region; wherein thestep of affixing the supporting substrate to the major surface of thefirst mirror stack includes depositing a supporting layer of heatconducting material on the major surface of the first mirror stack; andwherein the supporting layer of heat conducting material is notthermal-expansion matched to the compatible substrate.
 11. A method offabricating a long-wavelength vertical cavity surface emitting laser asclaimed in claim 10, wherein the step of depositing the supporting layerof heat conducting material on the major surface of the first mirrorstack includes depositing metal.
 12. A method of fabricating along-wavelength vertical cavity surface emitting laser as claimed inclaim 11 wherein the step of depositing metal includes depositing one ofcopper, silver, gold, nickel, and aluminum.
 13. A method of fabricatinga long-wavelength vertical cavity surface emitting laser comprising thesteps of: depositing a long wave-length active region on a compatiblesubstrate, the long wave-length active region having a first majorsurface; depositing a first mirror stack on the first major surface ofthe long wave-length active region so as to define a major surface ofthe first mirror stack; affixing a supporting substrate to the majorsurface of the first mirror stack; removing the compatible substrate toexpose an opposed second major surface of the long wave-length activeregion; depositing a second mirror stack on the second major surface ofthe long wave-length active region; and forming a light inlet openingthrough the supporting substrate for optically pumping thelong-wavelength vertical cavity surface emitting laser.
 14. A method offabricating a long-wavelength vertical cavity surface emitting lasercomprising the steps of: depositing a long wave-length active region ona compatible substrate, the long wave-length active region having afirst major surface; depositing a first mirror stack on the first majorsurface of the long wave-length active region so as to define a majorsurface of the first mirror stack; affixing a supporting substrate tothe major surface of the first mirror stack; removing the compatiblesubstrate to expose an opposed second major surface of the longwave-length active region; depositing a second mirror stack on thesecond major surface of the long wave-length active region; and formingat least one of the long wave-length active region and the second mirrorstack to provide index guiding for the long-wavelength vertical cavitysurface emitting laser.
 15. A method of fabricating a long-wavelengthvertical cavity surface emitting laser comprising the steps of:depositing an InP based long wave-length active region on an InP basedsubstrate, the long wave-length active region having a first majorsurface; depositing a metamorphic distributed Bragg reflector on thefirst major surface of the long wave-length active region so as todefine a major surface of the distributed Bragg reflector; affixing asupporting substrate to the major surface of the distributed Braggreflector; removing the InP based substrate to expose an opposed secondmajor surface of the long wave-length active region; and depositing asecond mirror stack on the second major surface of the long wave-lengthactive region.
 16. A method of fabricating a long-wavelength verticalcavity surface emitting laser as claimed in claim 15 wherein the step ofdepositing a metamorphic distributed Bragg reflector includes depositingalternate layers of Al_(x)Ga_(1-x)As and Al_(y)Ga_(1-y)As, where x in arange of from approximately 0.5 to 1 and y is in a range of fromapproximately 0 to 0.5.
 17. A method of fabricating a long-wavelengthvertical cavity surface emitting laser as claimed in claim 15 whereinthe step of depositing a metamorphic distributed Bragg reflectorincludes depositing alternate layers of microcrystalline silicon andmicro-crystalline silicon carbide.
 18. A method of fabricating along-wavelength vertical cavity surface emitting laser as claimed inclaim 15 wherein the step of depositing a metamorphic distributed Braggreflector includes depositing alternate layers of micro-crystallinesilicon and micro-crystalline aluminum oxide.
 19. A method offabricating a long-wavelength vertical cavity surface emitting laser asclaimed in claim 15 wherein the step of affixing the supportingsubstrate includes affixing one of a supporting layer of heat conductingmaterial and an InP based substrate.
 20. A method of fabricating along-wavelength vertical cavity surface emitting laser as claimed inclaim 15 wherein the step of affixing the supporting layer of heatconducting material includes plating metal.
 21. A method of fabricatinga long-wavelength vertical cavity surface emitting laser as claimed inclaim 20 wherein the step of plating metal includes plating one ofcopper, silver, gold, nickel, and aluminum.
 22. A method of fabricatinga long-wavelength vertical cavity surface emitting laser as claimed inclaim 15 wherein the step of depositing the second mirror stack includesdepositing one of a metamorphic distributed Bragg reflector and adielectric mirror stack.